Since f(x) = ax/(2x+3), to find f( f(x) ), replace the x=term in ax/(2x+3) wwith ax/2x+3):
f( f(x) ) = f( ax/(2x+3) ) = [ a( ax/(2x+3) ) ] / [ 2( ax/(2x+3) ) + 3 ]
= [ a2x / (2x+3) ] / [ 2ax / (2x+3) + 3]
Assume that x isn't -3/2: multiply both the numerator and denominator by 2x + 3 and simplify:
= a2x / ( 2ax + 3(2x+3) ]
= a2x / ( 2ax + 6x + 9 ]
Since f( f(x) ) = x: a2x / ( 2ax + 6x + 9 ] = x
Multiply both sides by 2ax + 6x + 9: a2x = x(2ax + 6x + 9)
Assuming that x isn't 0, divide both sides by x: a2 = 2ax + 6x + 9
---> 2ax + 6x = a2 - 9
---> 2x(a + 3) = (a + 3)(a - 3)
If a is not equal to -3, divide both sides by a + 3:
---> 2x = a - 3
and x = (a - 3) / 2
But, x could be zero (it checks in the original expression).
Also checking -3/2 and -3 -- they don't check.
So, the answer: either 0 or (a - 3)/2.